Soybean is the vegetable oil source produced in the greatest quantity worldwide. Since the composition of fatty acids contained in plant oil has a direct effect on the quality of the oil, modification of the composition of fatty acids is a crucial part of breeding, even in soybean. As regards fatty acid composition, generally, if saturated fatty acid content is high, oil quality will be stable. However, ingesting this sort of oil increases the level of low density lipoprotein cholesterol in the blood and is therefore not nutritionally preferable. Further, the polyunsaturated fatty acids like linoleic acid are problematic because they easily oxygenate, are unstable, and easily produce sludge when used as a biofuel.
As in the past, hydrogenation is practiced as a means of stabilizing polyunsaturated fatty acids. This method is, however, not economic because of the costs incurred. Hydrogenation is also not preferable because it produces trans-fatty acids which are believed to lead to risk of coronary disease as a by-product. In contrast, since monounsaturated fatty acids such as oleic acids are comparatively stable to oxidation and lower the blood cholesterol level, soybean rich in oleic acid is preferable both as a source of fuel and as a source of nutrition.
Generally, since the composition of the fatty acid obtained from generally distributed soybean seeds has an oleic acid content of 20 to 25% and a linoleic acid content of 50 to 57%, breeding was undertaken in order to increase the oleic acid content. However, products developed until the present do not contain a practically adequate content of oleic acid.
According to research conducted until the present, in order to increase oleic acid content in vegetable oil, it is reported that it is crucial to reduce the activity of endoplasmic reticulum omega-6 fatty acid desaturase (FAD2), which catalyzes the linoleic acid production process (nonpatent literature 1) by desaturating oleic acid. Therefore, the FAD2 gene that encodes this enzyme is a target during breeding. As FAD2 genetically-modified soybean for which gene modification technology is used, ultrahigh oleic acid soybean (line name: 260-05) in which the expression of the FAD2 gene is suppressed by the co-suppression method has been developed, being reported that oleic acid content has increased to approximately 80% or higher (patent literature 1 and non-patent literature 2). Further, studies of increasing oleic acid content have been reported (patent literature 2) in which the expression of the gene that encodes palmitoyl-ACP thioesterase, which is involved in adjusting the ratio of saturated fatty acid to unsaturated fatty acid, is reduced by genetic modification. However, since these high oleic acid soybean were genetically modified, they were finally rejected by consumers and are not being produced at the present for the purpose of food production.
Accordingly, when the purpose is to provide soybean or soybean oil that contains high oleic acid for consumption, it is preferable to provide high oleic acid soybean without using genetic engineering techniques. In the past, the present inventor prepared 2 types of mutant soybean, namely M23 and KK21, in which the functions of the GmFAD2-1a genes, one of the FAD2 genes, are completely destroyed by inducing mutation and in so doing succeeded in increasing the oleic acid content from approximately 25% to 50% or higher (patent literature 3 and nonpatent literature 1). However, it was difficult to increase the oleic acid content further without using genetic modification.